Boron nitride is chemically and thermally stable and further has excellent electrical insulating properties. Therefore, boron nitride is used as a lubricant and a releasing agent which are used at a high temperature and is also used as a heat resistant and corrosion resistant material or a member in the field of electricity as one of so-called new ceramics.
Heretofore, boron nitride has been industrially produced by: (1) a method comprising reduction nitriding boric acid, borates, boron oxides or the like with ammonia gas (optionally in the presence of ammonium chloride); and (2) a method comprising mixing boric acid, borates, boron oxides or the like with nitrogen-containing compounds and then reduction nitriding the mixture in nitrogen or ammonia gas.
Other than the above methods, a method comprising heating elemental boron in nitrogen or ammonia gas, a method comprising heating boric acid, borates, boron oxides or the like together with metal cyanides, a method comprising heating boron halides, boron hydrides or boron/metal compounds in ammonia gas, and a method comprising heating borides such as CaB.sub.6 in the presence of nitrogen are known. These methods, however, have disadvantages in that special starting materials are used, and are merely employed in laboratory.
The above-described two methods (1) and (2) which are industrially employed also have the problem that the degree of conversion is low. The reason thereof is that the melting point of boric acid, borates or the like is low and, therefore, those are liquid within a temperature range at which those are reduction nitrides with ammonia gas or an ammonia/nitrogen gas mixture, resulting in a poor contact between those materials and the gas.
In order to overcome the foregoing problem, several methods have been proposed and employed. One method is a method comprising adding calcium carbonate or calcium phosphate and optionally boron nitride as a carrier to boric acid or borates, reacting at a temperature of 900.degree. C. or more in such a manner that the surface of the carrier is covered with a thin layer of molten boric acid or borates to obtain a crude boron nitride, removing additives with, for example, hydrochloric acid, and then heat treating at 1,800.degree. C. to obtain powdered boron nitride. This method, however, requires a step of removing the additives, which greatly increases production costs. Furthermore, it is difficult to completely remove the additives.
Another method is a method comprising mixing nitrogen-containing compounds such as urea, dicyan diamide or melamine with, for example, borax and the reacting in an inert gas such as nitrogen or a reducing gas such as hydrogen while forming an intermediate which does not melt at a temperature of from 800.degree. to 1,200.degree. C. as described in Japanese Patent Application (OPI) No. 27200/72 (the term "OPI" as used herein means a "published unexamined Japanese patent application"). In this method, the ratio of the nitrogen-containing compound to the boric acid salt, for example, must be controlled to a narrow range and, furthermore, the mixing must be sufficiently and uniformly conducted. If the ratio is outside the specified range or the mixture is not uniform, the degree of conversion is reduced and unreacted compounds are left. As a result, the sponge-like product must be pulverized and washed with water to remove by-products and unreacted compounds and then again subjected to a heat treatment at 1,800.degree. C. in a stream of ammonia.
These methods involve a purification step where boron nitride formed by the high temperature reaction is pulverized to remove the additives and by-products, and a re-combustion step at a high temperature. Therefore, improvement is required from the standpoint of the steps and also the energy consumption. Boron nitride produced by the methods is usually in a flake-like crystal form and such a crystal form is difficult to control. This crystal form of boron nitride imposes limits on improvements in sintering properties of boron nitride, sintering of a mixture of boron nitride and other ceramic materials, utilization of boron nitride as a composite material, and so forth.